This present invention relates to water purification and, more particularly, to an improved device for determining water purity based on its TOC (or total organic carbon) value. The present invention also relates to an ultrapure water production system incorporating such TOC monitors that produces ultrapure water in a continuous and cost effective process.
High purity water is required for many purposes, including use in semiconductor device fabrication, medicine, biology, etc. Conventionally, there are a number of techniques for purifying water, such as ultrafiltration, reverse osmosis, distillation, sorption, ion exchange, deareation, ultraviolet oxidation. They are usually employed in combinations to provide the highest purity. Water contains various impurities such as particles, ions, microorganisms, organic carbons, dissolved oxygen. For example, if water for use in fabricating semiconductor devices is not purified enough to contain trace quantities of residual organics, they tend to leave the impurities on a wafer surface leading to a significant reduction in the production yield.
Various types of devices are available for monitoring the purity of an effluent water. One measurement commonly employed is specific resistivity in megohm-cm at 25.degree. C., a measure of ionic contamination. Pure water has a theoretical resistivity of 18.2 megohm-cm. The amount of particles present in water has also been used as a measure of water purity. Recently, a variety of so-called "TOC" monitors have been developed which measure the total amount of organic carbons present in water as a measure of water purity. The TOC monitors have enabled continuous monitoring of the quality of ultrapure water for use in a semiconductor device fabrication process. Since the organic carbons are generally electrically non-conductive, one type of TOC monitor utilizes ultraviolet or thermal oxidation to transform such organics into carbon dioxide for subsequent infrared analysis. However, this type of TOC monitor is relatively costly. Another type of TOC monitor which is simple in construction and relatively inexpensive measures the resistivity of water after oxidizing organic carbons into ionic organic acids by ultraviolet oxidation.
The TOC monitor of the latter type includes a pair of opposed electrodes for measuring the resistivity of the water flowing therebetween. Referring to FIG. 1 showing its principles of operation, the meter includes a pair of electrodes 10, 12 placed in water 14 in a beaker 16. This arrangement enables accurate resistivity measurement so long as water purity is at low levels. However, it is known in the art that as water purity approaches the theoretical level of 18.2 megohm-cm, the ultrapure water becomes adsorptive so that it takes up carbon dioxide present in the air, causing a significant drop in the water purity.
The adsorption of carbon dioxide can be minimized by placing the electrodes like electrodes 18 within closed conduits 20, 22, as shown in FIGS. 2 and 3. In the FIG. 3 arrangement, the electrodes 18 comprise two concentric tubes extending into the bent flow path. These arrangements, however, have the inherent shortcomings that, for water of high purity, measurement errors are significantly greater during very low flow rates as shown in the graph of FIG. 4. With the arrangements, accurate resistivity measurement is very difficult for ultrapure water having a resistivity above 5 megohm-cm and flowing at lower than 100 cc/min. This is because the ultrapure water passing between the electrodes would be at very small amounts during very low flow rate conditions. Further, bubbles are generated and released from the electrodes due to electrolysis, tending to affect the measurement accuracy and reliability. It would be desirable to provide an improved arrangement which effectively increases the amount of ultrapure water flowing between the electrodes and which efficiently removes the electrolysis caused bubbles to thereby permit an accurate resistivity measurement for ultrapure water flowing at very low rates.
It is therefore the principal object of the present invention to provide an improved device for determining water purity with a view to overcoming the above-noted disadvantages of the prior art.
It is another object of the present invention to provide an improved device which measures the TOC value of an effluent water as an indication of water purity and which permits an accurate and reliable measurement of water purity even at very low flow rates.
It is a further object of the present invention to provide an improved water purity determining device which minimizes the influence of electrolysis caused bubbles on measurements.
It is still further object of the present invention to provide an improved TOC monitor arrangement in which an electrically conductive, tubular section of the conduit serves as one electrode within which the other, cylindrical electrode extends concentrically forming a narrow, annular space for increased flow of ultrapure water to be measured.
It is still another object of the present invention to provide an improved TOC monitor arrangement in which the inner cylindrical electrode is axially movable within the outer tubular electrode to vary the cell factor of the electrodes for optimum measurement.
It is a further object of the present invention to provide a ultrapure water production system incorporating improved TOC monitors that produces ultrapure water in a continuous and cost effective process.
It is still further object of the present invention to provide a ultrapure water production system in which the TOC monitors are used to permit an early detection of the deterioration of ion exchange resins for purifying the water.